There are many applications for precision force measurement devices. One application is measuring acceleration. Other applications include measuring forces on gyroscopes, use in seismography, in microphones and geophones, controlling positioners, in inclinometers, surface-tensionometers, in flow and pressure measurement instruments, sensing forces on wind-tunnel models, in active vibration damping systems, and in dynamic balancing of rotating machinery.
Most force measuring devices are not of the force-balance type, which involves feedback position stabilization of a moving “sense” element. They do not use feedback control, but simply measure the free motion of an element experiencing external force and then relate this motion to the force applied. These are generally known as open-loop type devices, rather than closed-loop type devices of the present invention. Some production devices do use a piezoelectric element as a force sensor, always without feedback control. When the piezo elements deform due to applied force, they produce a voltage on attached electrodes, non-linearly related to that force. Most of these piezo-based instruments are “bearingless” structures—they indirectly measure position change due to external force applicationon a mass supported by elastically-deformable restraints. This results in very long life and good stability over time, as well as good shock and vibration resistance. However, most do not exhibit very high accuracy or repeatability. One percent accuracy would be considered very good, whereas most are in the three to five percent range.
The present invention is aimed at applications requiring more accuracy, generally well under one percent, along with the ability to withstand harsh vibration and shock environments. The invention is based on combining the force-balance feedback stabilization principle with a piezoelectric force generating element that generates its own rebalancing force in response to a voltage feedback control. This provides a number of significant improvements illustrated by describing their use in a common linear accelerometer force sensing application and in a pressure gauge that has a diaphragm supported to sense external pressure. Many existing open-loop accelerometers use the deflection of a piezoelectric element under acceleration forces to generate a voltage related to its deflection. U.S. Pat. No. 6,655,211 illustrates an example. Voltage is measured and converted to an acceleration, often using a calibration table stored in electronic memory to compensate for non-linearity in the sensing element. No open-loop pressure gauges are believed currently known using piezoelectric elements for readout.
There are two broad classes of piezo-based force generating elements—“blocks” and “bimorphs”. Both are used in accelerometers, but the bimorphs are more common. The blocks are solid pieces of material with conductive electrodes deposited on them in locations chosen to maximize electrical-to-motion coupling for a given deformation (“mode”). Practical motion is very small, usually under a few micro-inches. Bimorphs are thin sandwiches made of a metal core between two layers of piezo material bonded to it. They are usually made as strips, much narrower than they are long. The electrodes are the core and thin conductive sheets deposited on the outside faces of the piezo layers. With proper design, these sandwich structures can bend under applied voltages. The two long-direction ends move up or down relative to the center of the strip. Notions are typically relatively large. If one end is clamped down, the opposite end can move many thousands of micro-inches. Both blocks and bimorphs can be stacked on top of one another. For blocks, this results in taller structures that provide more free-end motion at lower drive voltage than would a single block of the same height with electrodes only at its ends. For bimorphs, this results in a stiffer sandwich capable of exerting more force at its tip for a given voltage than a single bimorph. Both blocks and bimorphs can be clamped down at one end, or, in the middle, or somewhere else. This results in one, two or more ends that are free to move under force application. Several suppliers produce usable piezoelectric materials. One vendor in the United States offering a particularly-wide variety of materials and designs is PI (Physik Instrumente) of Auburn, Mass.
Piezoelectric materials are somewhat reciprocal. They will change shape in response to applied electric fields (usually produced by voltages on electrodes). They will also produce similar voltages on those electrodes when mechanically deflected the same amount by outside forces. This latter effect is what most piezoelectric open-loop accelerometers employ.
All piezoelectric materials also exhibit inherent hysteresis—a failure to return exactly to a prior position/voltage output after experiencing a large deflection in one direction. They are also inherently non-linear—voltage and motion are not directly proportional. However, the force exerted by the element is much closer to linear with applied voltage. This hysteresis and the inherent nonlinearities are what limit the accuracy and repeatability of current piezo accelerometers. They can be numerically corrected, but this is rarely done due to the complexity of the calculations and required memory. For those devices designed to measure constant accelerations (like gravity), a third error source is the draining off of charge from the piezo material by the measuring circuits over time.
Present open-loop piezo accelerometers are essentially voltmeters hooked to electrodes on a piezo material. When the piezo material deforms under acceleration forces, the voltage produced on the electrodes is read.